After oil has been removed from a subterranean dolomite reservoir by primary recovery methods such as water or gas drive or pumping or by natural migration, a very large amount of low saturation oil still remains in the formation. At this stage, it is fairly common to employ various secondary recovery measures in an effort to extract at least some of the remaining oil. One type of secondary recovery process that has been widely used is called "in-situ" combustion where a fire is started at the bottom of one well which burns carbonaceous reservoir materials (kerogen) in the rocks in the presence of an oxidizing medium such as air. Inherent in this process is the production of flue gas which includes carbon dioxide, nitrogen and carbon monoxide. Although it is generally recognized that carbon dioxide will make low saturation oil more movable by swelling the oil and lowering its viscosity, flue gas has a low efficiency respecting displacement of oil in the reservoir because the carbon dioxide is in a diluted form.
Various processes have been proposed to generate sufficient carbon dioxide in an in-situ combustion process that would make secondary recovery economically feasible. For example the Sharp U.S. Pat. No. 3,174,543 discloses in-situ combustion of natural reservoir materials together with introduction of a driving fluid which is miscible with the Co.sub.2. The driving fluid, the gas phase and the oil are intended to be forced toward a production well. An electrical resistance heater is used to initiate burning at a temperature of about 500.degree.. The Speller, Jr. U.S. Pat. No. 3,964,545 discloses the injection of air to cause an oxidation reaction with carbonaceous material in the formation to produce CO.sub.2, which would make oil in the surrounding area more movable. Kamath U.S. Pat. No. 4,465,135 discloses injection of ozone and/or oxygen to support in-situ combustion which produces CO.sub.2 that would increase the movability of the oil adjacent the fire front. Gilliland U.S. Pat. No. 3,408,082, although not directed to a secondary recovery process, proposes in-situ reporting of oil shale near the surface by injecting CO.sub.2 which has been heated to a relatively high temperature at the surface. The combustion zone also is pressurized to a range of about 500-1,000 psi to avoid burning limestone and dolomite rocks. The Bridges et al U.S. Pat. No. 4,821,798 discloses an electrical heating system to increase the temperature of the oil and thereby reduce its viscosity. The casing strings are used as parts of the electrical circuit. The Gibson et al U.S. Pat. No. 4,336,864 proposes forming an underground, rubbilized cave between an injection well and a recovery well by burning limestone to create calcium oxide which then is contacted with water to produce a slurry of calcium hydroxide. The calcium hydroxide is then flushed out to create void spaces. Hydraulic fracturing or other means is employed to cause the remaining materials to cave in and form the rubbilized zone. Thus although production of CO.sub.2 in various secondary recovery processes is known, most of these processes are aimed at liberating CO.sub.2 by burning the natural kerogen materials or oil that remain in a reservoir rock after primary completion methods have been exhausted, or have reached their economic limit.
A general object of the present invention is to provide a new and improved secondary recovery process where dolomite rock in an oil bearing formation is subjected to a controlled heating to high temperatures to dissociate the same into other materials including CO.sub.2 which makes the oil in surrounding rocks more movable.